1. Field of the Invention
The present invention relates to a vehicle braking system comprising: a master cylinder in which a master piston with its back turned to a boosted hydraulic pressure chamber is slidably housed in a casing; a backup piston which is slidably housed in the casing with its front face turned to the boosted hydraulic pressure chamber and with its backward limit fixed, to push the master piston directly from behind in response to operation of a brake operating member when the hydraulic pressure of the boosted hydraulic pressure chamber decreases; and pressure regulating means which applies output hydraulic pressure of a hydraulic power source to the boosted hydraulic pressure chamber after regulating the output hydraulic pressure according to brake operating input from the brake operating member; the master cylinder being connected to wheel brakes.
2. Description of the Related Art
Such a vehicle braking system is disclosed, for example, in Japanese Patent Application Laid-Open No. 4-283157.
However, in the conventional vehicle braking system, seal diameter of a backup piston on a casing is set larger than seal diameter of a master piston on the casing. Consequently, when the hydraulic pressure of a boosted hydraulic pressure chamber decreases due to a defect of a hydraulic power source, if the backup piston moves forward in response to operation of a brake operating member, the amount of volume decreased in the boosted hydraulic pressure chamber caused by the forward movement of the backup piston will exceed the amount of volume increased in the boosted hydraulic pressure chamber caused by the forward movement of the master piston, resulting in increase in the hydraulic pressure of the boosted hydraulic pressure chamber to speed up the movement of the backup piston if no measure is taken. To avoid such a situation, an emergency control valve is interposed between the boosted hydraulic pressure chamber and a reservoir to communicate the boosted hydraulic pressure chamber with the reservoir as the hydraulic pressure of the hydraulic power source decreases. This not only complicates the configuration, but also increases the number of parts.